Conventionally, there is known a road-vehicle communication system that includes a roadside apparatus installed on a road and a vehicle-mounted device mounted on a vehicle and performs two-way wireless communication between the roadside apparatus and the vehicle-mounted device. In this road-vehicle communication system, for example, a short-range communication system called DSRC (Dedicated Short Range Communication) is adopted.
The vehicle-mounted device is capable of performing the short-range wireless communication with the roadside apparatus and receiving information provision from a center apparatus via the roadside apparatus. Specifically, only in a period when the vehicle is within a communication range of the roadside apparatus, two-way communication between the vehicle-mounted device and the roadside apparatus is possible. Content information is provided from the center apparatus to the vehicle-mounted device via the roadside apparatus in this period.
The road-vehicle communication system can provide the vehicle-mounted device with, as the content information, information for supporting traveling (traveling support information) such as information indicating that there is an obstacle such as a stalled vehicle ahead on a road or there is a junction ahead. Therefore, the road-vehicle communication system is extremely effective for preventing traffic accidents.
In a road-vehicle communication system on a road having up and down traffic lanes, roadside apparatuses are respectively installed in the up traffic lane and the down traffic lane. The up and down traffic lanes are discriminated to provide traveling support information effective in the respective vehicle lanes. For example, the traveling support information is provided for each of the up and down lanes, for example, before a curve with poor visibility, a junction where a merging vehicle tends to be overlooked, and a section where various kinds of information such as a traffic situation that occurs ahead and weather conditions by a highway radio or the like.
In such a case, the traveling support information is provided for each of the up and down traffic lanes by installing a communicable range such that a radio wave originated from the roadside apparatus does not reach the opposite lane. However, because of leakage, irregular reflection, or the like of the radio wave, in some case, the traveling support information is provided to a vehicle on the opposite lane that is originally excluded from provision targets.
In particular, in a vehicle-mounted device (e.g., an utterance type vehicle-mounted device) that does not have a navigation function and cannot specify the present location (a traveling lane, a traveling direction, etc.) of a vehicle on which the vehicle-mounted device is mounted, basically, received information is directly reproduced (uttered). Therefore, if information that should not originally be provided is received and output, a driver is confused and safety of driving is likely to be hindered.
As a technique for preventing, for example, leakage of radio waves in the road-vehicle communication system, for example, there is Patent Document 1. A technique described in Patent Document provides, in an ETC system as an example of the road-vehicle communication system, a radio wave absorber that can suppress irregular reflection of radio waves in a communication area of a transmission and reception antenna and leakage of radio waves from the communication area.    Patent Document 1: Japanese Patent Application Laid-Open No. 2002-237719